I use to have a double width garage with a proper full pitched roof.
I contacted the council & was informed as long as I didn't change the external appearance I wouldn't need planning permission to alter the inside.
Fortunately I had double doors for both sides, ie, four doors rather than the up & over style.
This allowed myself to use 3 feet across almost the whole width in from these doors to use as storage.
The almost the whole width refers to the fact I installed a full size stair case on the right side up to what became an office over the studio below.
I used thermal plaster board against the walls, basically they are plaster board with styrene on one side & then silver foil, no not the oven type, proper building regulation foil.
This foil goes against the brick work as a barrier against damp & cold.
The boards were fixed to the brick with plaster board adhesive, I had to get a plaster to skim finish the boards though.
I put a floating floor in & then tongue & groove floor boards so no nails showed, Oh & more of the styrene board between the floating floor sections for further insulation.
I put in a skylight to the roof line for the office but put it to the garden side so as not to alter the look from the public view.
I put in central heating taken from the house, yes the garage was a detached building.
I took the pipes over the side gate & insulated them & took a separate electric supply with its own consumer unit.
In all this cost 4K with me doing all except the plastering.
Just to add there was already a side door to access the garage without needing to go through the main doors which is how I managed to effectively shut this section off & create the front as storage.